Isothermal Martensite Formation

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Isothermal (i.e. time dependent) martensite formation in steel was first observed in the 40ies of the XXth century and is still treated as an anomaly in the description of martensite formation which is considered as a-thermal (i.e. independent of time). Recently, the clarification of the mechanism leading to isothermal kinetics acquired new practical relevance because of the identification of isothermal martensite formation as the most likely process responsible for enhanced performances of sub-zero Celsius treated high carbon steel products. In the present work, different iron based alloys are chosen to investigate time dependent martensite formation. Among them, a Fe-11wt%Ni-0.6wt%C model alloy and Fe-1.6wt%Cr-1wt%C (AISI 52100), Fe-17wt%Cr-7wt%Ni (AISI 631) and Fe-16wt%Cr-5wt%Ni (AISI 630) commercial steels.
The investigation was performed with in situ magnetometry, dilatometry, synchrotron XRay diffraction and ex situ electron backscatter diffraction. Magnetometry and dilatometry were applied to investigate the kinetics of the transformation. Synchrotron X-Ray diffraction was applied for the determination of lattice strains in austenite. Electron backscatter diffraction was applied to characterize the microstructure of the material and the morphology of martensite. The investigation of the kinetics yielded information on the mechanism responsible for time dependent martensite formation, which was indicated as thermally activated growth of athermally nucleated martensite. The investigation of lattice strains provided fundamental information on the state of stress in the material and clarified the role of the strain energy on martensite formation. Electron backscatter diffraction revealed that the microstructure of the material and the morphology of martensite were independent on the cooling conditions during sub-zero Celsius treatment.
Irrespective of the morphology of martensite (lath or lenticular), it was observed that decelerations and accelerations of the transformation occur. This characteristic of the transformation was explained as a consequence of a partially athermal, partially thermally activated character of the transformation.
Original languageEnglish
Place of PublicationKgs. Lyngby
PublisherDTU Mechanical Engineering
Number of pages314
ISBN (Print)978-87-7475-375-9
Publication statusPublished - 2014


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